Several areas of chemical technology involve the transfer of granular solids from a non-oxidative gas contacting zone to an oxidative gas contacting zone, while preventing the transfer of gases between the respective zones. The retorting of oil shale followed by combustion of coke on the spent shale is a prime example. Other examples include the use of moving beds of granular absorbents or catalysts to effect chemical separations or conversions, wherein more or less continuous removal of coke or other deactivating deposits from the solids is carried out by combustion. In these and other gas-solids contacting processes, one widely used method for isolating the gases in the respective contacting zones involves the use of expensive and elaborate lock vessels, valves, star feeders, slide valves and the like, through which the solids are allowed to pass while restricting the flow of gases. These devices are subject to failure through rapid wearing of moving parts, and tend to produce fines through crushing or abrading of the solids. Another method which has been utilized involves the injection of an inert sealing gas into the transfer zone between the two contacting zones, a portion of which passes through the combustion zone, and another portion through the treating zone. Conventionally, this seal gas system is controlled by differential pressure controllers, which are very sensitive and subject to upsets, especially when pressure differentials between the two contacting zones may fluctuate due to surges in gas flow rates. My control system to be described hereinafter avoids all of these major difficulties.
In the use of an inert seal gas, the difficulties involved in the use of differential pressure controllers could obviously be avoided if off-gases from the respective contacting zones could be produced at a constant rate while still maintaining constant pressures. However, in most contacting processes, including shale retorting, the rate of production of product gases in the retorting or other treating zone may vary substantially from time to time, as well as the rate of production of flue gases from the combustion zone. This renders difficult the production of off-gases from the respective contacting zones under constant flow control and pressure, while still insuring that some of the inert seal gas will at all times flow through each contacting zone. Further complications arise when the generally most desirable sealing gas, steam, it utilized. Shale retorting off-gases, as well as off-gases from many catalytic contacting processes, must be cooled to condense out liquid hydrocarbons and this also brings about condensation of steam. Constant flow control over product off-gases would thus not be responsive to steam passing through the retorting or other contacting zone. The flow control system provided herein is uniquely adapted to the use of steam as the sealing gas.
In broad aspect my control system described herein finds utility in gas-solids contacting processes wherein a moving bed of granular solids is contacted in an enclosed treating zone at elevated temperatures with a stream of non-oxidizing gas, resulting in the production of a net off-gas therefrom, the rate of production of said net off-gas varying between a maximum expected value, M, and a minimum expected value, m, and wherein solids from the treating zone are transferred through an enclosed transfer zone to an enclosed combustion zone wherein they are contacted at elevated temperatures with a stream of oxidizing gas. Briefly summarized, to prevent the transfer of gases between the treating zone and the combustion zone, the critical features of my control system are as follows:
(1) substantially constant pressures are maintained in the treating zone and combustion zone by exhausting flue gas from the combustion zone in response to pressure detected at some point in the transfer zone; PA1 (2) a portion of the net off-gas, or make gas, generated in the treating zone is, after cooling, withdrawn from the process at a substantially constant predetermined flow rate, x, which is less than the value, m; PA1 (3) steam is injected into a lower portion of the transfer zone at a substantially constant predetermined flow rate, y, which is greater than M - m; and PA1 (4) at some point between the treating zone and the point of steam injection at (3) above, a hot mixed gas stream is withdrawn from the transfer zone at a substantially constant, predetermined flow rate, z, wherein z is greater than M - x, but less than y + m - x.
By operating in this manner the mixed gas stream at (4) will always comprise the net off-gas not withdrawn at step (2) above plus some steam, and some of the steam injected at step (3) will always be forced to pass through the combustion zone. As applied to shale retorting, the control scheme insures high yields of oil and a high BTU make-gas, prevents contamination of product gas with flue gas, prevents high BTU gas from the retort from entering the combustion zone, prevents air from the combustion zone from entering the retort, and maintains steady control of system pressure. The system is also unaffected by variations in pressure drop through the beds of shale in the retort and combustor.